I have been thinking/looking at ballast tank designs and i was wondering if one of my ideas might be plausible, so far the best one i have seen is this one on youtube:
http://www.youtube.com/watch?v=oBvYJAROLDU
its and ok design but because of the center shaft it would double in length when empty. i also relized another problem. should a ship rely on this design, the empty space where air is in the back of the ballast tank would be open to the rest of the ship. So.... should it get shot and it empty the tanks to surface, well it would still fill up from the rest of the ship and no matter what become a full tank, thus definatly dooming the entire ship 
.
anyway.....
i was thinking spacewise to be more efficient, a design where it would have something like a syringe with only one opening on the big end, and have a centrifugal pump there? I think it would (Possibly) be more compact than most mechanical designs i have seen. it could have a small 1 way valve to prevent water going back in 
I found this picture to show the pump, you could have the discharge going up into a small port under the wooden deck so it would be pushed out under the deck (or somewhere else if you wanted) and the suction would be coming from the syringe. What do you guys think?